Incompatible Trace Element Abundances Research Articles

Laser ablation–inductively coupled plasma–mass spectrometry and tephras: A new approach to understanding arc-magma genesis

Research Article| December 01, 1999 Laser ablation–inductively coupled plasma–mass spectrometry and tephras: A new approach to understanding arc-magma genesis C. J. Bryant; C. J. Bryant 1Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Geology and Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar R. J. Arculus; R. J. Arculus 1Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Geology and Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar S. M. Eggins S. M. Eggins 1Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Geology and Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (12): 1119–1122. https://doi.org/10.1130/0091-7613(1999)027<1119:LAICPM>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C. J. Bryant, R. J. Arculus, S. M. Eggins; Laser ablation–inductively coupled plasma–mass spectrometry and tephras: A new approach to understanding arc-magma genesis. Geology 1999;; 27 (12): 1119–1122. doi: https://doi.org/10.1130/0091-7613(1999)027<1119:LAICPM>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The development of laser ablation–inductively coupled plasma–mass spectrometry has revolutionized the analysis of tephras by providing (1) an efficient and precise method for determining abundances of a wide variety of trace elements at low concentrations in individual glass shards and (2) assessment of geochemical heterogeneities within individual ash horizons. This development is important for petrogenetic studies of intraoceanic arc systems, where tephras provide the most complete temporal record of magmatism. Results from the Izu-Bonin and Mariana arc systems indicate that despite close geographical proximity and similar tectonic evolution, they contrast strongly in terms of geochemical evolution since 35 Ma. Whereas the Mariana tephras have exceptional compositional diversity, ranging from low-K (Oligocene), to high-K (Miocene), and subsequently medium-K compositions (Pliocene-Quaternary), the Izu-Bonin arc has been dominated by low-K compositions throughout. The Mariana increases in K are paralleled by increases in abundances of incompatible trace elements and by increased values of diagnostic ratios (e.g., Nb/yb and Th/yb) regarded as monitors of potential mantle-source fertility. The relative uniformity of Nb/yb and Nb/Zr ratios in Izu-Bonin tephras indicates that cyclic processes of backarc basin development and mantle depletion do not necessarily induce large-scale temporal geochemical variations in the associated arc. Temporal variability within the Mariana arc, and its divergence from the Izu-Bonin arc ca. 13 Ma, can be traced to a major injection of subducted sediment in the Mariana system at this time. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Major- and trace-element systematics and isotope geochemistry of Cenozoic mafic volcanic rocks from the Vogelsberg (central Germany): Constraints on the origin of continental alkaline and tholeiitic basalts and their mantle sources

The Cenozoic basaltic province of the Vogelsberg area (central Germany) is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar Ba/Nb, Ba/La and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, Zr/Nb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher Ba/Nb, Zr/Nb, La/Nb, but lower Ce/Pb ratios and lower Nd isotope compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower (granulite facies) crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites.

Emplacement and Crystallization Time for the Bushveld Complex

The Bushveld Complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years. For example, the Columbia River Basalts (Hooper, injections of magma, which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988) were erupted in the period 17–12 Ma, with minor eruptions for a further 5 my, although most outpouring significantly before the addition of the next one. To constrain the emplacement and crystallization times, a thermal model is presented occurred within the first 2 my. It is now recognized that large intrusions were not emplaced in a single pulse, but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition (and subresult from multiple magma injection. The question is how rapidly were magma chambers, such as the BC, traction). Such modelling indicates that magmas injected into the Bushveld Complex were emplaced within 75 000 years. At that filled and how much magma was involved. Answers are relevant to the dynamics of melt production, storage and time injection into the Complex ceased. The volume of rock in the Eastern and Western limbs is 370 000–600 000 km. However, transport in the mantle and crust. This paper describes a thermal modelling technique (not previously applied a quantitative evaluation of the Cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers) which can be used to analyse this process, and to obtain an estimate of the emplacement accounted for in the preserved rock sequence. Similarly, an evaluation of the incompatible trace-element abundances, such as those for Zr time. To present this model it is necessary to discuss the stratigraphy, size, and connectivity of the different limbs and K, suggests that the chamber was open and that large volumes of differentiated magma escaped. The volume of magma therefore of the Bushveld Complex, and to consider the extent of tapping of the magma chamber as well as its filling. greatly exceeded the preserved volume of cumulate rocks, giving an estimated magma volume of over 1× 10 km. An average The term ‘Bushveld Complex’ has been given several meanings in the literature, and according to the South emplacement rate of 13 km/year is indicated by these calculations. African Commission on Stratigraphy (1980) includes not only the ultramafic–mafic layered rocks, but also the sills beneath the intrusion, volcanic rocks which pre-date the

The effect of crustal contamination on ultrapotassic magmas with lamproitic affinity: mineralogical, geochemical and isotope data from the Torre Alfina lavas and xenoliths, Central Italy

Torre Alfina, a Quaternary volcano located in the northernmost part of the Roman Province, is characterized by two main types of mafic ultrapotassic lavas with slightly different chemical, isotopic, and mineralogic characteristics. Mica-rich enclaves, peridotitic and granulitic xenoliths, and upper crustal xenoliths are hosted by the lavas. Both of the lavas have the assemblage olivine+clinopyroxene+sanidine+phlogopite with mafic phases showing insufficient Si+Al cations to completely fill the tetrahedral sites, as is typical of lamproitic minerals. Other features are also transitional between lamproite mineralogy and Roman-type mineralogy. Chemically, the host lavas have high Mg/Mg+Fe values (71–77) and compatible trace element contents (e.g., Ni=245–249 ppm), as well as high K 2O (5.7–7.5 wt.%) and incompatible trace element abundances. Furthermore, they have high initial 87 Sr/ 86 Sr (0.7159–0.7165) and low initial 143 Nd/ 144 Nd (0.51211–0.51212) values. Passing from the most primitive through the more evolved lavas, the Sr isotopic ratios, (Tb/Yb) N ratios, and SiO 2 and Al 2O 3 contents increase, whereas the abundances of both the compatible and incompatible trace elements decreases. Crustal xenoliths are represented by shale, sandstone, gneiss, micaschist, and granulitic rock fragments. They have variable chemical compositions with 87 Sr/ 87 Sr and 143 Nd/ 144 Nd values, calculated at the time they were trapped by the magma, in the ranges 0.7136–0.7176 and 0.51204–0.51209, respectively. Digestion of crustal rocks, such as those included in the lavas, is not able to drive the magmas from Roman-type to lamproitic compositions. Mass-balance calculations show that extensive crustal contamination of lamproitic parental magma can produce the observed chemical, isotopic, and mineralogical characteristics of the Torre Alfina rocks. Since no crustal rocks with incompatible trace element abundances as high as those of lamproitic rocks are common at the earth's surface, lamproitic magmas assimilating crustal rocks become diluted in incompatible trace elements, and enriched in silica and alumina.

Trace element abundances of high-MgO glasses from Kilauea, Mauna Loa and Haleakala volcanoes, Hawaii

We performed an ion-microprobe study of eleven high-MgO (6.7–14.8 wt%) tholeiite glasses from the Hawaiian volcanoes Kilauea, Mauna Loa and Haleakala. We determined the rare earth (RE), high field strength, and other selected trace element abundances of these glasses, and used the data to establish their relationship to typical Hawaiian shield tholeiite and to infer characteristics of their source. The glasses have trace element abundance characteristics generally similar to those of typical shield tholeiites, e.g. L(light)REE/H(heavy)REEC1 < 1. The Kilauea and Mauna Loa glasses, however, display trace and major element characteristics that cross geochemical discriminants observed between Kilauea and Mauna Loa shield lavas. The glasses contain a blend of these discriminating chemical characteristics, and are not exactly like the typical shield lavas from either volcano. The production of these hybrid magmas likely requires a complexly zoned source, rather than two unique sources. When corrected for olivine fractionation, the glass data show correlations between CaO concentration and incompatible trace element abundances, indicating that CaO may behave incompatibly during melting of the tholeiite source. Furthermore, the tholeiite source must contain residual garnet and clinopyroxene to account for the variation in trace element abundances of the Kilauea glasses. Inversion modeling indicates that the Kilauea source is flat relative to C1 chondrites, and has a higher bulk distribution coefficient for the HREE than the LREE.

Cross-arc geochemical variations in volcanic fields in Honduras C.A.: progressive changes in source with distance from the volcanic front

A geochemical traverse across Honduras reveals the heterogeneity of the mantle underneath Central America. Alkali basalts from Lake Yojoa (170 km behind the front) have low 87Sr/86Sr but high La/Yb, and elevated incompatible trace element abundances, consistent with derivation from a normal mid-ocean ridge basalt source mantle via low degrees of melting. These lavas lack evidence for an enriched source thought to be intermingled with normal mid-ocean ridge basalt source mantle beneath most of Central America. The amplitude of the subducted slab signature decreases smoothly with distance from the volcanic front. Lavas from Zacate Grande, the area nearest to the volcanic front (17 km behind the arc), display large ion lithophile element enrichment and high field strength element depletion indicating the involvement of subducted material in magma genesis. Components of subducted material are not evident in lavas from Lake Yojoa, the area furthest from the arc. Basalts and basaltic andesites from Tegucigalpa, 102 km behind the volcanic front, are geochemically intermediate between those of Lake Yojoa and Zacate Grande. The lavas from Tegucigalpa show a decreased influence of the subduction component, and are affected by assimilation-fractional crystallization processes at shallow depths. The gradual decrease in the subducted component from the volcanic front to Zacate Grande, Tegucigalpa and finally Lake Yojoa contrasts with the abrupt decrease documented for southeast Guatemala, the only other area in Central America where a cross-arc transect has been studied.

Crustal Origin for Peralkaline Rhyolites from Kenya: Evidence from U-Series Disequilibria and Th-Isotopes

The Olkaria complex is a recent, peralkaline rhyolite field in the Kenya Rift Valley close to the axial region of the Kenya Dome. U–Th disequilibrium was measured by α-spectrometry in whole rocks and mineral separates from seven geographically and compositionally distinct groups (centres) of rhyolites. Forty-three whole-rock samples show variable Th/U ratios (2.8–6.2) and a large range of (238U/232Th) ratios (0.5–1.1); 79% of the rhyolites show U excess. Rocks from some centres plot entirely to the left or right of the equiline, whereas some centres straddle it. Internal isochrons give U–Th ages of between 14.6+2.2−2.1 and 36.2+2.6−2.6 ka (2σ) respectively for the Gorge Farm centre and 50.5+7.9−7.3ka for the Broad Acres centre. These ages, interpreted as phenocryst crystallization ages, are older than eruption ages by 103–104 yr. The youngest centre displays (226Ra/230Th) > 1, indicating that Ra–Th fractionation has taken place <8000 yr bp. There is a positive correlation between (226Ra/230Th) and (238U/230Th) ratios for rocks of the youngest centre, indicating that the Ra enrichment and the U enrichment probably occurred during the same event. Closed-system fractionation of observed mineral phases cannot alone explain the U-series disequilibria although it may have contributed to other compositional features of the rhyolites. The degree of U enrichment is related to major and trace element variations in the rhyolites as a whole, in that there is a correlation with peralkalinity and thus with incompatible trace element abundances, in particular Nb, Rb and Zr. There is a good correlation between (238U/230Th) and pre-eruptive F contents but not pre-eruptive H2O contents, consistent with previous suggestions that the rhyolites formed by halogen-fluxed melting of the crust. Compositional variations between groups of rhyolites are related to heterogeneity of the crustal source rocks, degree of partial melting(and thus residual mineralogy) and the composition and abundance of metasomatic fluids.

北海道日高帯，幌満マントルダイアピル内での全岩主化学組成と微量成分組成の挙動

Petrochemical studies based mainly on major and trace element whole-rock XRF analyses have been made on the Horoman and the Nikanbetsu peridotite complexes, Hokkaido, Japan. Incompatible trace element abundances in peridotite samples were obtained by carefully examining the XRF lower limit of detection. The Horoman complex is divided into Lower and Upper Zones on the basis of geological and petrological features. There is a systematic variation between Mg# [100×Mg/(Mg+Fe)] and whole-rock chemical compositions in both zones. There are, however, several differences between the plagioclase lherzolite samples from the Upper Zone and those of the Lower Zone. First, the range of Mg# of the plagioclase lherzolite from the Upper Zone is more variable than that of the Lower Zone. Some samples from the Upper Zone have higher Mg# than the maximum values for plagioclase lherzolite from the Lower Zone. Such samples are significantly poor in Al2O3, CaO, TiO2, Sr and Y. Second, there is a gap in TiO2 content in the compositional trend for the plagioclase lherzolite from the Upper Zone. This observation enables the division of the plagioclase lherzolite into TiO2-rich and-poor types. These results indicate that the compositional variation of the plagioclase lherzolite from the Upper Zone was caused by pervasive partial melting and variability in the extent of segregation of the partial melt. By contrast, the plagioclase lherzolite from the Lower Zone is inferred to have kept its original composition, which is consistent with petrographic evidence for no or very minor partial melting. Spinel lherzolite and harzburgite from the Horoman complex are rich in Sr and Ti relative to Zr and Y. Plagioclase lherzolite from the Upper Zone occurring close to the segregation vein has a character more enriched in melt component and was probably derived by addition of a melt component to the primitive mantle composition. The Nikanbetsu complex may have the same origin as the Horoman complex because of their similarity in major and trace compositions.

Petrology and geochemistry of the ultrapotassic rocks from the Sabatini Volcanic District, central Italy: the role of evolutionary processes in the genesis of variably enriched alkaline magmas

The Sabatini Volcanic District (SVD) is a large volcanic field characterised by the lack of any major volcanic center. Its activity, spread over a wide area, started at 0.6 Ma and developed through five main phases, during which several calderas and the Bracciano lake volcano-tectonic depression were formed. All the volcanic rocks belong to the Roman-type ultrapotassic series (HKS), ranging from leucite tephrites to leucite and haüyne phonolites. Although the major- and compatible-element contents indicate a single series of evolution, there are differences in the incompatible trace-element abundances. A high-Ba series (HBaS) has been distinguished from a low-Ba series (LBaS), with the former also enriched in all other incompatible elements (e.g., REE, Nb, Zr, Th) except Rb. The HBaS rocks are plagioclase-free, leucite-bearing lavas and were abundantly outpoured from the Bracciano volcanoes during the third and fifth phase of activity. Plagioclase- and phlogopite-bearing rocks constitute the LBaS and were erupted during the other phases generally from smaller and eccentric volcanic centers. The initial 87Sr/ 86Sr values are higher in the HBaS rocks and do not vary significantly with magma evolution (0.71047–0.71080), but cover a wider range in the LBaS rocks (0.70944–0.71038), with the lowest Sr isotope ratios occurring in the least evolved lavas. The higher Ca content in the olivine and Ti and Al IV in the clinopyroxene, and the lower ulvöspinel content of the Ti-magnetites of the HBaS rocks suggest an evolution at lower pressure and higher temperature for this magma. The observed petrologic characteristics suggest that the HBaS magma evolved at lower depths by processes of refilling, tapping, fractionation and probably assimilation (RTFA), where the crystallisation rate of clinopyroxene+leucite±olivine dominates over the input rate of the fresh magma. The LBaS magma evolved at slightly higher pressure, in separate and small magma bodies, by fractional crystallisation of clinopyroxene+plagioclase±phlogopite±olivine that was often associated with crustal assimilation (AFC). It has been suggested that RTFA processes with high input rate/crystallisation rate ratios could also be responsible for the differentiation between the HBaS and LBaS. The different processes of evolution undergone by the HBaS and LBaS could have been related to the different volumes of magma rising from the source.

Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe, Tanzania

This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K2O/Na2O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An82–90), clinopyroxene (4.5–9.5 wt% Al2O3), and olivine (Fo79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P2O5) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P2O5/Al2O3, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO2-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle.

Chemical evolution of a large mafic intrusion in the lower crust, Ivrea‐Verbano Zone, northern Italy

The Ivrea‐Verbano and adjacent Strona‐Ceneri zones have been described collectively as a section through the continental crust. While resident in the lower crust, amphibolite to granulite‐facies paragneiss of the Ivrea‐Verbano Zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the Mafic Complex. Growth of the Mafic Complex involved hypersolidus deformation in an extensional environment. Isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle‐derived melts that were extensively contaminated by crustal material. Previous investigations determined that the contaminant was fingerprinted by 87Sr/86Sr &gt; 0.71, δ18O = 10–12.5‰, and a positive Eu anomaly. In the present study, the contaminant is also shown to have been enriched in Ba with respect to Rb and K. Charnockites associated with paragneiss septa in the lower part of the Mafic Complex have the appropriate chemistry to be samples of the contaminating material. These chemical features can be explained by melting of granulite‐facies paragneiss, which had previously been depleted in K and Rb by an earlier melting event. The Ba enrichment in the core of the Mafic Complex can be modeled by a replenishment‐tapping‐fractional‐crystallization (RTF) process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by Ba‐rich charnockite. Very high Ba/K in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa. In contrast to the Mafic Complex, the chemistry of coeval granites in the adjacent Strona‐Ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event. Significantly, the incompatible trace element abundances in the Mafic Complex and Strona‐Ceneri granites are similar to model compositions for the lower and upper crust, respectively.

Ultra-depleted primary melt included in an olivine from the Mid-Atlantic Ridge

MODELS of magma genesis at mid-ocean ridges1, together with recent experimental data2 and observations of trace element abundances in clinopyroxenes from abyssal peridotites3, suggest that small-volume melt fractions can be efficiently extracted from the melting mantle. As shown in ref. 3, residues of this type of melting (fractional melting) display extremely low abundances of incompatible trace elements and extreme fractionation amongst them, especially at advanced stages of the process because of the compounded effects of differences in the partition coefficients. If this process operates beneath mid-ocean ridges, one would expect to sample melts that are correspondingly depleted and fractionated in trace elements. Indeed, the existence of very depleted melts in mid-ocean ridges was predicted previously4–6 in order to explain the presence of magnesian pyroxene and calcic plagioclase in mid-ocean-ridge basalts. We report here the discovery of melt that has major and trace element characteristics consistent with these predictions4–6, occurring as an inclusion in an olivine phenocryst in a typical mid-ocean-ridge basalt from the Mid-Atlantic Ridge. Although our preferred model for the origin of this 'ultra-depleted' melt is critical (continuous) melting, we cannot at this stage rule out other models. Our results underscore the importance of trapped melt inclusions as recorders of the processes involved in melting and melt extraction, and also as pointers to primary melt compositions.

The origin of Mount St. Helens andesites

Mount St. Helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites. Petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system. Mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants. The mafic lavas include: (a) transitional, olivine+plagioclase basalts with low K 2O and incompatible trace-element abundances; (b) calc-alkaline, olivine+plagioclase + clinopyroxene basalts enriched in K 2O, TiO 2, and incompatible trace elements; and (c) calc-alkaline. olivine+plagioclase basaltic andesites with incompatible trace-element contents transitional between the two basalt types. Intermediate lavas include (a) tholeiitic, two-pyroxene andesites, (b) calc-alkaline, plagioclase+two-pyroxenes±olivine±amphibole mafic andesites (56–59% wt.% SiO 2), and (c) calc-alkaline, plagioclase+two-pyroxenes+amphibole high-silica andesites (61–62 wt.% SiO 2). Eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages, and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches. It is unlikely that any of the andesites (or dacites) are derived by fractional crystallization of the recognized basaltic variants. Formation of the andesites simply by contamination (or assimilation-fractional crystallization) of basaltic magma is also improbable. More plausibly, the andesites represent mixing between basaltic and dacitic end-member magmas, each of which may be somewhat heterogeneous or vary in composition with time. In this model, efficient mixing must occur in some parts of the magma plumbing system, while some conduits or storage reservoirs must be effectively isolated.

Compositional evolution of high-temperature sheared lherzolite PHN 1611

The evolution of “fertile” mantle has been studied by proton microprobe (PIXE) analysis of minerals of a high-temperature sheared xenolith from the Thaba Putsoa kimberlite in Lesotho, southern Africa. Analyzed elements include Ni, Cu, Zn, Ga, Sr, Y, and Zr. Garnets are homogeneous in Ni and Zn but have rims enriched relative to cores in Zr and Y. Compositions of olivine neoblasts define intergranular gradients of Fe, Zn, and Ni; Fe-rich olivine is relatively Zn-rich but Ni-poor. Although individual clinopyroxene grains are nearly homogeneous, clinopyroxene associated with Fe-rich olivine is relatively Fe- and Zn-rich but Sr- and Cr-poor. The trace-element abundances and compositional gradients constrain the processes of peridotite enrichment and the thermal history. Enrichment of Zr, Y, and Fe in garnet rims documents infiltration of a silica-undersaturated melt. The Fe-rich olivine compositions and the Zn and Fe gradients establish that the xenolith was sampled from near a melt conduit. Mechanical mixing of inhomogeneous peridotite and melt infiltration may have been concurrent. Because garnets appear homogeneous in Ni, mantle temperature changes affecting PHN 1611 occurred before or over a longer period than the melt infiltration. Measured and calculated abundances of many incompatible trace elements in the rock are similar to those proposed for primitive mantle. Calculated chondrite-normalized abundances of Sr, Ti, Zr, and Y are like those of appropriate REE. Enrichment processes in PHN 1611 proceeded at unusually high recorded temperature and in the apparent absence of minor phases common in lower-temperature metasomatized rocks, but similar processes may be common. In particular, mechanical mixing near mantle dikes may frequently occur. These enrichment mechanisms may produce xenolith compositions that resemble some proposed for primitive mantle but that have different implications for mantle evolution.

Geochemistry of tholeiites from Lanai, Hawaii

Lanai is the third smallest of the fifteen principal subaerial shield volcanoes of the Hawaiian hotspot. This volcano apparently became extinct during the shield-building stage of volcanism, as shown by the absence of both alkalic cap and post-erosional lavas. Major and trace element analyses of 22 new samples collected primarily from 3 stratigraphic sections show that Lanai tholeiites span a large range in composition. Some Lanai lavas are unique geochemically among Hawaiian tholeiites in having the lowest abundances of incompatible trace elements of any Hawaiian lavas and well-developed positive Eu anomalies. The geochemical characteristics of these low-abundance Lanai tholeiites are not the result of alteration, differences in mantle source modal mineralogy, the presence of residual accessory mantle phases or fractional crystallization of such phases, assimilation of depleted [MORB] wall-rock, or accumulation/resorption of phenocrysts or xenocrysts. Incompatible trace element ratios (e.g., Nb/La, Nb/Th, La/Th, La/Hf, Ce/Pb) in Lanai tholeiites span considerable ranges and form coherent trends with each other and with absolute abundances of these elements. Large variations in La/Sm, La/Yb, and absolute REE abundances at constant MgO suggest that Lanai tholeiites formed by variable amounts of partial melting. However, large ranges in incompatible element ratios cannot be explained solely by variations in partial melting of a geochemically homogeneous source, but must reflect geochemical heterogeneities in the Lanai source. Partial melting modeling indicates that the mixed Lanai source is probably LREE-enriched [i.e., (La/Yb)CN>1]. One component in the Lanai source, exemplified by the low-abundance tholeiites, has markedly lower REE/HFSE, Th/HFSE, alkali/HFSE, and Ce/Pb ratios than other Lanai or Hawaiian tholeiites and may indicate the presence of recycled residual subduction zone materials in the Hawaiian plume source. The positive Eu anomalies that characterize the low-abundance Lanai tholeiites are not the result of plagioclase accumulation or assimilation but are a feature of this source component. Progressive temporal geochemical variations in Lanai tholeiites from 2 stratigraphic sections indicate that the source composition of these lavas probably evolved over time. This change could have resulted from a progressive decrease in the extent of partial melting of the Lanai source. The compositional variability of Lanai tholeiites suggests that geochemical heterogeneities in their source are larger than the scale of partial melting. Lanai tholeiites could not have formed by smaller degrees of partial melting of plume material than did the larger-volume Hawaiian shields. Therefore, volume differences between Hawaiian shields must be controlled primarily by differences in the volume of supplied plume material rather than by differences in the degree of partial melting. The premature cessation of eruptive activity at Lanai may be attributed to relatively large degrees of partial melting of a small plume.

An appraisal of melting processes and the Galapagos hotspot: Major- and trace-element evidence

The variation of Nd, Sr, and Pb isotopes in lavas from the Galapagos Islands is thought to be the result of contamination of a mantle plume by the upper mantle. In order to evaluate the control of source mixing on the compositions of the lavas. the major- and incompatible trace-element compositions of parental melts are calculated by normalizing their values in aphyric, unaltered lavas to MgO = 8% along calculated liquid lines of descent. The Galapagos lavas have approximately half the worldwide major-element compositional range of MORBs. Of all the elements and ratios evaluated, only volcano-averaged K and La/Sm correlate with the isotopic ratios. This must indicate that differences in the melting process, such as depth, extent of melting, or both overwhelm any compositional difference between the plume and upper mantle. Several of the volcanoes have exceedingly large ranges in the abundances of incompatible trace elements, but the range of isotopic values from single volcanoes is small and best explained by large ranges in the extents of melting. Inferred parental lavas also have a large range of Si and Fe concentrations, which may result from extraction of the melts over a range of depths. Element abundances and isotopic ratios have geographic patterns which indicate that magmas from the center of the archipelago are derived by relatively lower average extents of melting, deeper average extraction depths, and less contribution of plume material. In general, however, the mantle beneath the central islands experienced wide ranges in temperature (extent of melting), and the melts were extracted from widely different depths. The lavas of the western and northern islands are more uniform, probably controlled by consistent temperatures in the underlying mantle and depth of extraction. These relationships are not explainable in terms of simple two-stage melting and mixing models. The compositions of parental Galapagos magmas are controlled by a complex sequence of processes which have different effects on the isotopic, major-element, and trace-element compositions of the magmas.

Evolution of calc-alkaline magmas at the early Quaternary Battle Ax volcano, Western Cascade Range, Oregon

Battle Ax volcano is a dissected composite cone of early Quaternary age, located 33 km northwest of Mt. Jefferson in the Western Cascade Range of Oregon. It evolved during three major eruptive episodes, each of which produced lavas with a distinctive set of petrographic, geochemical and paleomagnetic features. The oldest rocks (Series I) consist of olivine-bearing basaltic andesites and two-pyroxene andesites, with the earliest erupted lavas generally being most mafic. The intermediate-age rocks (Series II) are a compositionally uniform suite of two-pyroxene andesites. The youngest rocks (Series III) consist mainly of hornblende andesite, but also include dacitic lavas and rhyolitic tuff. The first eruptions during the Series III episode produced the most silica-rich rocks and succeeding eruptions discharged progressively more mafic lavas. Rocks comprising one eruptive series cannot be related to those of another series by any simple fractionation or mixing process, owing mainly to intra-series differences in the abundances of incompatible trace elements. Compositional variations among the Series I lavas can be attributed primarily to fractional crystallization of plagioclase, olivine (or orthopyroxene), augite, and magnetite (POAM). The most mafic basaltic andesites in this series probably accumulated small amounts of olivine. The Series III magma evolved mainly by fractionation of plagioclase, amphibole, and orthopyroxene, ±magnetite, ±apatite. Compositional differences among the porphyritic andesites of Series III reflect different amounts and proportions of accumulated xenocrysts derived from hornblende-gabbro cumulates. The most silicic rocks in Series III are probably mixtures of fractionated magma and crustal components. Stratigraphic relations among the Series III lavas indicate they represent magmas erupted from progressively deeper levels in a compositionally zoned reservoir.

Petrological and geochemical constraints on the genesis of Mesozoic–Cenozoic magmatism of King George Island, South Shetland Islands, Antarctica

Petrological and geochemical data are reported for a series of Late Cretaceous-Middle Miocene volcanic, hypabyssal and intrusive rocks from King George Island (KGI) and from nearby Ridley Island, South Shetland Islands. Major element data indicate a calc-alkaline, basic to intermediate composition for the analysed samples. Although emplaced on a continental margin, the KGI rocks generally display low abundances of incompatible trace elements, close to those typically observed in calc-alkaline suites erupted in intraoceanic island arcs. A few samples have a significant negative Ce anomaly. Many incompatible elements define smooth positive trends on interelemental variation diagrams which suggests that magmas erupted at different times on KGI maintained a rather constant composition in terms of incompatible element ratios. Geochemical modelling, based on Sr isotope ratios and incompatible element ratios, suggests that the primary calc-alkaline magmas of KGI were all generated in an upper mantle modified by addition of small amounts of pelagic sediments dragged down by subduction processes.

Compositional changes in Trans‐Pecos Texas Magmatism Coincident with Cenozoic stress realignment

The composition and inferred sources of Cenozoic magmas in Trans‐Pecos Texas changed abruptly between 32 and 28 Ma, coincident with a fundamental change in stress regime. Magmas emplaced between 48 and 32 Ma comprise extended differentiation suites beginning with relatively evolved basaltic magmas. These rocks have low Nb and Ta compared to Zr, Hf, La, Ba, and K, characteristics typical of continental volcanic arcs. However, the Texas rocks are more alkalic, have higher concentrations of incompatible elements, and have higher ratios of Nb and Ta to Zr, Hf, La, Ba, and K than is typical of rocks from arcs near trenches. These differences reflect relative position within the Cordilleran arc. Trace element models involving partial melting in the mantle wedge combined with fractional crystallization and assimilation of upper and lower crust can account for most of the observed trace element concentrations and ratios. Trans‐Pecos magmatism may have involved smaller degrees of partial melting than is typical of arcs located closer to the subduction zone. Magmas emplaced after 31 Ma were either bimodal, alkali basalt‐rhyolite, or exclusively basaltic. They have high absolute abundances of incompatible trace elements and high ratios of Nb and Ta to Zr, Hf, La, Ba, and K. The mafic rocks are typical of continental rift or ocean island basalts. Trace element models using Nb, Ta, Y, Yb, Hf, and Zr reproduce the range of concentrations and ratios observed in these rocks and require very small degrees of partial melting, variable amounts of fractional crystallization, but little or no assimilation. The change in magma compositions and sources is best illustrated by a drop in Zr/Nb, from between 18 and 6.25 in the earlier, arc rocks to between 6.25 and 2 in the later, rift‐related rocks. Stress regime appears to play an important role in the process of magma generation and evolution. The change in trace element compositions indicative of tectonic setting supports interpretations of paleostress data that activity up to 31 Ma occurred in a continental volcanic arc, whereas later activity occurred in a setting of intraplate extension. The magmatic change may have been simultaneous with the change in stress regime or may have lagged by as much as 3–4 m.y.

Geochemistry and petrogenesis of Tethyan ophiolites from northern Argolis (Peloponnesus, Greece)

Tethyan ophiolitic sequences from the northern part of the Argolis Peninsula in northeastern Peloponnesus (Greece) contain MORB-like basalts and rocks with boninitic characteristics. The former include basalts with enriched to depleted light REE patterns. The boninitic rocks show many similarities to those of Cenozoic age from the Western Pacific - as well as to the low-Ti ophiolites - including high contents of Mg, Ni and Cr and low abundances of Ti and incompatible trace elements. The boninites that are the most depleted in Al, Ca, Ti and heavy REE, display the highest relative enrichment in light REE and Zr. The boninitic rocks were probably formed in a back-arc/inter-arc setting where a rising MORB-source diapir facilitated melting of a progressively more depleted upper mantle peridotite that had been modified by Zr- and light REE-rich fluids. The compositional variations of MORB-like basalts are attributed to dynamic partial melting of the rising mantle diapir.